The obese Zucker rat is widely used as a model for obesity, yet the macroscopical energetics related to its fatty state have not been satisfactorily partitioned. Determinations will be made of total energy balance of adult obese and lean Zucker rats for measurement of food energy intake, body compositional change, total heat production, and total heat loss with separation of total heat production into physical activity, thermogenesis, and the thermic effect of food ingestion. Direct and indirect calorimetry as well as food intake assessment and the measurement of change in body composition will be employed. Use of both direct and indirect calorimetry will reveal the transient thermal responses of the lean and obese animals to different ambient temperatures. Differences in body heat loss will be noted and associated with colonic temperature changes. Adult animals will be used initially in order to avoid the complicating factor of growth in the young. We wish to test the maintenance energetics of the adult lean and obese Zucker rat; it is our hypothesis that there is a fundamental difference in the energy balance of these animals, particularly when cold or otherwise stressed. A null result of our investigations would suggest that the fundamental difference in energy balance is to be found in the preadult maturation phase of growth of the rats and growing and maturing animals must be studied--a change in emphasis that we are prepared to make. The results of the proposed research may be significant in identifying factors associated with genetically linked obesity and in appropriate suggestions for treatment of such a condition.